Journal of Earth Sciences and Geotechnical Engineering, vol. 4, no. 3, 2014, 77-94
ISSN: 1792-9040 (print), 1792-9660 (online)
Scienpress Ltd, 2014
Geoelectrical and Geotechnical Evaluation of Foundation
Beds at Naze, Owerri Southeastern Nigeria
Sabinus I. Ibeneme 1, Kalu K. Ibe1 and Nwankwo I. Stephen 2
Abstract
A foundation study has been undertaken at a proposed building site located at about 3km
east of Owerri, along Aba-Owerri road, between the Nekede Mechanic village and the
Naze/Nekede road junction. The aim of this study is to examine the Geoelectrical and
Geotechnical parameters that can be used to evaluate the stratigraphy, nature, structural
disposition/integrity and competence of the shallow section of the subsurface for
construction purposes and building development. The Vertical Electrical Sounding (VES),
using schlumberger configuration and soil analysis techniques were adopted. A total of
six (6) Vertical Electrical Sounding (VES) stations were occupied and this was
complemented with geotechnical analysis of six soil samples collected at all the VES
points down to a depth of 2m within the study area. Quantitative interpretations of the
VES curves were carried out using partial curve matching technique and computer
iteration technique. The investigation delineated four geologic layers which include the
Sandy and Organic-rich top soil, Iron-stained sand, Medium-grained sand and Gravely
sand. The topsoil composed mostly of sand and laterite with resistivity generally greater
than 350Ωm. The layer (topsoil) with thickness that varies from 1.4–5.8m constitutes the
layers within which normal civil engineering foundation is founded. There are no
indications of any major structures such as faults, fractures and lineaments down to a
depth of 40m that could be deleterious to engineering construction in the area. All the
determined geotechnical parameters of the subsoil fall within the specification
recommended for foundation material by FMWH, 1972 and are generally increasing
towards the southern part of the area. From the results, we conclude that soil units
between 0.0-2.0 meters (0-6.6 feet) are characterized by very high bearing capacity of
values ranging from 381 to 524 KN/M2. The deduction from the above is that, the topsoil
formation may be rated as relatively good as a foundation material. The foundation of the
proposed civil structure can be hosted by this formation.
Keywords: Bearing Capacity, Foundation, Geoelectric, Lineament, Resistivity,
Schlumberger.
1
Corresponding Author, Tel: +234(0)8026182505, Department of Geosciences, Federal University
of Technology, Owerri, Imo State, Nigeria.
2
Department of Architecture, Abia State University Uturu, Abia State, Nigeria.
78
Sabinus I. Ibeneme et al.
1 Introduction
Engineering structures are designed and constructed with long life expectancy [1].
Generally, most problems of structural failure are often associated with improper
founding of foundations and poor quality of building materials. However, inadequate
knowledge of the physical parameters governing the competency of the soil supporting
engineering structures has also been identified [2]. Consequently, the primary
consideration is the foundation and the capacity of the foundation bed to sustain both the
dead and live load imposed by the overlying structure. Hence the stability of the
foundation and the superstructure supported by the foundation depends on the bearing
capacity of the geologic materials underlying the site [3]. Usually, civil and building
engineers prefer drilling, cone penetrometer test and some other geo-technical methods in
assessing the strength of materials for the support of infrastructures such as roads,
buildings and dams [4]. However, it is believed that the integration of geophysical
methods have become indispensable in general foundation studies, since they offer a fast,
cheap and cost effective method of evaluating the geotechnical competence of sub-soils
and rocks for building foundations as well as providing subsurface information at
reasonable cost, locate critical areas for test drilling and thus eliminate less favorable sites
[5]. Consequently, an efficient sequence of foundation investigation should therefore
embrace surface geological survey, subsurface geophysical investigation and the
traditional insitu engineering/laboratory tests, in order to design the most suitable
foundation for the proposed structure. Therefore, it is imperative to complement these
with cost effective geophysical method which are commonly applied in engineering site
investigation ([6], [7]). Several works have established the relevance of application of
geophysical methods in the investigation of geomaterials underlying engineering
foundation sites ([8]; [9] and [10]), and the methods have been established to play
complementary roles in geotechnical studies, besides the fact that they are less expensive
and non-invasive. Dipole-dipole array has been used to investigate the causes of road
failure along Ilesa-Akure-Benin federal highway [11]. It was reported that the road failure
was due to the clay content of the topsoil and the heterogeneous nature of the sub-base
and sub-grade material on which the road was constructed. Combined geophysical and
geotechnical methods have been used to study the sub-soil conditions and the electrical
properties of the soil which may have effect on the foundation of the proposed switch
station facility for telecommunication site [12]. It was observed that areas with low
resistivity have soil that can lead to severe corrosion and high concentration of dissolved
salts in the soil. The choice of the geophysical method is usually determined by the
geologic set up and the existence of significant contrast in the physical properties of the
subsurface layers ([13], [14]). In other to emphasis importance of predevelopment survey,
Geoelectrical and geotechnical investigations were carried out in this project to examine
those parameters that can be used to evaluate the stratigraphy, nature, structural
disposition/integrity and competence of the shallow section of the subsurface for
construction purposes and building development. Results of the study shall aid the
structural engineer in his design of appropriate structures that command professional feat.
This is informed by the spate of collapsed structures which necessarily are not due to
material problems but due to improper design, stemming from lack of proper foundation
elements; a condition that spurred this research.
Geoelectrical and Geotechnical Evaluation of Foundation Beds
79
2 Location and Accessibility
The Building site is located at about 3km east of Owerri, along Aba-Owerri road, between
the Nekede Mechanic village and the Naze/Nekede road junction (figure 1). It is located
approximately on the following co-ordinates Latitude 5o 27.430’N to 5o 27.446’N and
Longitude 7o 02.760’E to 7o 02.790’E. The site is readily accessible along Owerri-Aba
road, Egbu-Naze road and Nekede-Owerri new road.
Otamiri River
Figure 1: Location Map of Building Site (owerri-Aba Road)
3 Geology of the Study Area
Naze area is located in an area underlain by Benin Formation. The units are made up of
sandy to gravelly sands without any shale or swelling clays. In general, Benin Formation
spans from Miocene to Recent. It is the youngest of Niger Delta sediments. Its thickness
is about 6,000ft, and very little hydrocarbon accumulation has been associated with the
Benin Formation. The Benin Formation comprises the top part of the Niger Delta clastic
wedge, from the Benin-Onitsha area in the north to beyond the present coastline [15]. The
Benin formation consists of massive continental sands and gravels, it underlain
gradationally by the delta front paralic lithofacies. The top of the formation is the recent
sub aerially - exposed delta top surface and its base extend to a depth of 4600 feet. The
base is defined by the youngest marine shale. Shallow parts of the formation are
composed entirely of non-marine sand deposited in alluvial or upper coastal plain
environments during progradation of the delta [16]. Although lack of preserved fauna
inhibits accurate age dating, the age of the formation is estimated to range from Oligocene
to Recent [15]. Benin Formation covers the following areas: Benin City, Warri, Rivers
State, Cross River State, Abia state (Part of Umuahia, and Aba), Imo State ( some part of
Anara, Amaimo, Ikeduru, Atta Village) it covers the entire owerri area with its outcrops at
Ihiagwa behind Federal University of Technology Owerri.
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Sabinus I. Ibeneme et al.
Figure 2: Topographic map of the study area showing the sampling points
Figure 3: Geological map of the study area
4 Materials and Methods
The ABEM Terrameter model SAS 4000 with an in-built digital display and recording
system was used in the field reading operation. It has three operating modes (Resistivity,
IP, SP) in one complete unit. It is expandable to 2D/3D imaging and borehole tomography
for a broad range of near surface geophysical applications. Scopes of applications, data
acquisition speed, and accuracy have defined the architecture of the ABEM Terrameter
SAS 4000 model. Core electronics, such as the Signal Averaging System (a.k.a Signal
Stacking) from which the ABEM Terrameter SAS derives its name, deliver accurate and
dependable results at maximized speed thereby reducing field time. It has inbuilt
productivity software, incorporates a sophisticated signal-averaging (or signal stacking)
Geoelectrical and Geotechnical Evaluation of Foundation Beds
81
algorithm where consecutive readings are taken automatically and the results are averaged
continuously to improve the accuracy of the measurements. Rechargeable 12-volt batteries
coupled to the equipment provide the energy for the equipment operation. Three (3)
traverses were established across the study area (Figure. 1). Two (2) Vertical Electrical
Sounding (VES) stations were occupied along each of the traverses. A total of 6
soundings were carried out using the Schlumberger configuration. The electrode spacing
(AB/2) was varied from 1.5m to 95.0m, ensuring approximately 63 meters depth of
investigation. The cross over distance is encountered at 10.5m, 14m, and 42m. The cross
over position is the position of the potential electrode where voltage readings become so
small that they make very significant change in the resistance reading to be recorded [17].
They are done by keeping the current electrodes fixed and the potential electrodes are
moved at a specific distance in logarithmic steps in a progressive manner away from the
reference point in order of 1.5m, 2.0m, 3.4m, 4.5m, 6.0m, 8.0m, 14.0m, 18.0m, 34.0m,
42.0m, 55.0m, 72.0m, and 95.0m. Field readings in Ohms were reduced to apparent
resistivity values using the Schlumberger Equation:
ℓa
=
where
ℓa =
a
=
b
=
R
=
𝜋𝜋(a2/b – b/4) R
(1)
apparent resistivity in Ohm-m
AB/2, the Half Current Electrode Separation in meters
Potential electrode separation in meters
Meter Reading in Ohms
The layout is shown in figure 4
C2
C1
P1
P2
b
O
AB/
(a)
AB/
(a)
VES
Figure 4: The Schlumberger Electrode Configuration Used.
The apparent resistivity values were plotted against electrode spacing (AB/2) on a
bi-logarithmic graph sheet to generate depth sounding curves. The field curves were then
inspected visually for identification of the curve type. Partial curve matching was carried
out on the field curves. The interpretation results (layer resistivity and thicknesses) were
82
Sabinus I. Ibeneme et al.
fed into computer for 1-D computer assisted interpretation involving Zohdy software [18].
The final interpreted results were used for the preparation of geoelectric sections and
maps. The co-ordinate of each of the sounding stations in Universal Traversal Mercator
(UTM) was recorded with the aid of the “GARMIN 76CSx” personnel navigation
geographic position system (GPS) unit. Geotechnical measurement of insitu soil materials
were carried out to ascertain their engineering properties. Six (6) disturbed soil samples
were collected at different locations. The exercise involved opening of pits to about 1m in
each of the six (6) locations studied and collecting samples with auger at between 1.5 and
2.2m (). It is suggested that at this depth range, the organic-rich top soil may have been
by-passed and the virgin soil studied. The six locations are shown in fig. 1. These samples
were preserved in polythene bags and transported to the laboratory. The natural moisture
content of the samples collected from the field was determined in the laboratory within a
period of 24 hours after collection. This was followed by air drying of the samples by
spreading them out on trays in a fairly warm room for four days. Large soil particles
(clods) in the samples were broken with a wooden mallet. Care was taken not to crush the
individual particles. Methods of testing soils for engineering parameters were conducted
in accordance with [19] for all the soil samples collected. The tests include Natural
moisture content, Bulk Density, Triaxial compressive strength, and California Bearing
Ratio (CBR) test. Several softwares with different analytical modules were used in the
interpretations. These include Geosoft Strata 3 version, Surfer 11 and EQS Geostatistical
software version 6.1.
Plate 1: One of the researchers collecting soil samples at intervals of 1.5-2.2m
5 Results and Discussion
5.1 Geoelectric Results
The electrical resistivity method of geophysical prospecting using Vertical Electrical
Sounding (VES) technique was utilized to map the subsurface layers to a maximum depth
of about 40m. The VES curves generated are shown in figure 5 below. From the
geoelectric section, the four major geoelectric layers were Sandy and Organic-rich top soil,
Iron-stained sand, Medium-grained sand and Gravely sand, as obtained from the result of
Geoelectrical and Geotechnical Evaluation of Foundation Beds
83
the partial curve matching which was refined by computer iteration. These are tabulated
in the table 1 below. The results of interpreted results of electrical resistivity survey have
made it possible to map the area from the ground surface to depth of about 40m. The
major geoelectric sequences delineated were: Sandy and Organic-rich top soil,
Iron-stained sand, Medium-grained sand and Gravely sand. The first layer is made up of
topsoil (Sandy and Organic-rich) which has resistivity values ranging from 369 to 16300
ohm – meter. The thickness of the layer is between 1.4 and 5.8m. Beneath the topsoil lies
the Iron-stained sands which are mainly wet with resistivity values varying from 19700 to
21300 ohm-meter. The thickness ranges from 2.2 to 4.1m. The third layer is made up of
the Medium-grained sand that has resistivity values ranging from 5650 to 121000
ohm-meter with thickness of 4 to 18m. The fourth layer (gravely sand occupies the
remaining column with its shallowest top observed at 13m in VES3 (figures 6 and 7).
Table 1 below shows the summary of the results. Within the first layer down to a depth of
approximately 5.8m, the northern part of the study area has the lowest resistivity values (<
1000 ohm-meter) which is an indication of a silty environment. Towards the southern part
of the study area, we have a very high resistivity value reaching more than 16000
ohm-meter within this first layer (figure 8).
ℓ (Ωm)
Depth(m
2280
6750
1690
77100
2.2
4.8
17.7
Model Interpretation
Figure 5: Typical Modeled curve of the Study Area
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Sabinus I. Ibeneme et al.
Figure 6: Geoelectric Sections of VES 1, 2 and 3 of the Study Area
Figure 7: Geoelectric Sections of VES 4, 5 and 6 of the Study Area
Geoelectrical and Geotechnical Evaluation of Foundation Beds
85
Map of resistivity variations with depth at the study area is shown in figure 8 above. Within
the top soil down to a depth of about 5.8m (Layer 1), high resistivity values were observed
within the southern part of the study area. This indicates lack of clay bearing rocks within
these localities and as such depicts competence for load bearing. The deeper one goes to a
depth of about 25m, resistivity increases towards the east and further down to a depth of
approximately 40m and below, resistivity increases towards the north (figure 8). These
spatial variations in resistivity with depth could be ascribed to the thick deposition of
gravely sands within these localities and along the river bed which the Otamiri river
incised.
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Sabinus I. Ibeneme et al.
Table 1: Summary of the Geoelectrical Interpretation along the Dam axis
VES NO
LAYER PARAMETERS
Layer 1
Layer 4
Layer 3
Layer 2
ρ(Ohm-m)
Lithology
Depth(m)
ρ(Ohm-m)
Lithology
Depth(m)
ρ(Ohm-m)
Lithology
Depth(m)
ρ(Ohm-m)
Lithology
VES 1
Depth
(m)
2.6
2280
4.8
6750
Iron-stained
sand
17.7
1690
Medium-grained
sand
40.0
77100
Gravely sand
VES 2
5.8
3280
22.7
121000
9000
Gravely sand
-
-
-
3.9
16300
12.5
13400
Medium-grained
sand
Medium-grained
sand
40.0
VES 3
40.0
1190
Gravely sand
-
-
-
VES 4
1.4
1690
16.6
14600
12700
Gravely sand
-
-
-
4.7
1140
8.8
19700
Medium-grained
sand
Iron-stained
sand
40.0
VES 5
22.0
7840
Medium-grained
sand
40.0
12000
Gravely sand
VES 6
4.5
369
Dry
sandy
and
organic
rich soil
Sandy
top soil
Dry
sandy
and
organic
rich soil
Sandy
top soil
Sandy
and
organic
rich top
soil
Sandy
top soil
14.6
5650
Medium-grained
sand
40.0
548
Gravely sand
-
-
-
Geoelectrical and Geotechnical Evaluation of Foundation Beds
87
N
Layer 1
(Top Soil)
5
N
Layer 2
25
N
Layer 3
40
Depth
(m)
Figure 8: 3D Model of resistivity variations at 5, 25 and 40m depth in the study area
5.2 Geotechnical Results
Table 2 below shows the results of the geotechnical analysis carried out on the soil samples.
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Sabinus I. Ibeneme et al.
Table 2: Summary of Geotechnical Test Results
Bore Hole Depth (m) Natural
Bulk
Angle of Cohesion
No
Moisture
Density
Shearing
(KN/m2)
3
Content
(Mg/m )
Resistant
(%)
(0 )
SS-01
2.00
11.8
2.27
17
70
SS-02
2.00
11.0
2.28
20
74
SS-03
2.00
11.3
2.26
21
73
SS-04
2.00
10.2
2.23
20
65
SS-05
2.00
11.5
2.24
22
63
SS-06
2.00
12.4
2.28
22
70
Bearing
Capacity
(KN/m2)
381
485
510
427
472
524
The natural moisture content of tested soil samples ranges from 10.2 - 12.4%. It increases
from value of 11.8% in location SS-01 to an all high value of 12.4% in location SS-06,
a-southward increase in the value. This shows that the moisture content of the soil in the
area is relatively low at its natural state. Moisture variation is generally determined by
intensity of rain, depth of collection of sample and texture of the soil [20]. The northwestern
part of the study area shows the lowest moisture content value (figure 9). This could be as a
result of increased porosity and permeability as the soils could not retain appreciable
amount of water. This is because unconsolidated soils loose moisture very quickly on
exposure to heat as a result of little or no matrix. Low water retaintivity, high porosity and
permeability make the soil to be easily washed away; a condition that favours gully
development as is the case within the northern and northwestern parts of the study area.
N
Figure 9: Natural Moisture Content variation in the Study Area
Geoelectrical and Geotechnical Evaluation of Foundation Beds
89
N
Figure 10: Bulk Density variation in the Study Area
Bulk density range from 2.23 X 103kg/m2 in location SS-04 to value of 2.28 X 103kg/m3
in location SS-06. Bulk density decreases towards the northwestern part and increases
towards the eastern and southern parts of the area (figure 10). For shear angle, the value
ranges from 170 in location SS-01 to a maximum value of 220 in locations SS-05 and
SS-06 (figure 11). Cohesion is a function of silt or clay fraction. Cohesion values are
lower in locations SS-04, SS-05 and SS-06. However, mean value of 69.1KN/M2 was
obtained. From cohesion values, the northern and northwestern parts of the study area can
be said to contain low percentage of fines (figure 12). Fines content are known to
contribute to the strength of a soil as their small sizes and mineralogy increase the bond
between the grains. The fewer fines a soil has, the greater the ease with which moving
water can detach the particles. Thus the low value observed within the northern part of the
area indicates relative lack of strength; a condition that favours gully development as
observed in the area.
N
Figure 11: Angle of shearing Resistance variation in the Study Area
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Sabinus I. Ibeneme et al.
N
Figure 12: Cohession variation in the Study Area
From the result, no value of bearing capacity is less than 380KN/M2. This implies greater
sand fraction and competence of the soil at 2m depth. High values of 472, 485, 510 and
524 KN/M2 are recorded for samples from locations SS-05, SS-02, SS-03 and SS-06.
However, these values are typical of areas of overriding sandy fractions. From the contour
map of bearing capacity values, it shows that to the north of the study area (west of
Otamiri river), there is low bearing capacity and to the south of the study area (east of
Otamiri river -Naze) there is a high bearing capacity (figures 13 and 14). There is a
possibility of soft matter at the river bank to the west hence some treatment must be made
to the soil at the northern and northwestern parts of the study area.
N
Figure 13: Contour image map of bearing capacity values (KN/m2)
Geoelectrical and Geotechnical Evaluation of Foundation Beds
91
N
Figure 14: 3 D image map of bearing capacity values (KN/m2)
The topsoil constitutes the layer within which normal Civil Engineering foundation is
founded. The layer is composed of laterite and sandy materials. Foundation competence
of the topsoil can be qualitatively evaluated from layer resistivity and geotechnical
parameters [21]. According to [7], the higher the layer resistivity value, the higher the
competence of the delineated topsoil units. The Federal Ministry of Works and Housing
[22] says the higher the geotechnical parameters of a soil, the lesser the competence of the
soil as a foundation material. In the study area, the values recorded are lower and falls
within recommended value and thus accounts for the high competence of the soil as a
good foundation material.
5.3 Comparison between Geological, Geoelectrical and Geotechnical Results
The site is located in an area underlain by the Benin Formation. The relatively high
Bearing capacity of over 300 KN/m2 is correlated with overriding pebbly samples from
the borehole (figure 15) drilled and logged at the site by the researching team to provide
water in the area. This is equally correlated with high resistivity values obtained in figure
8 down to a depth of about 5.8m (within the top soil zone). The units here are made up of
sandy to gravely sands without any shale or swelling clays. From the log shown in figure
15, there is overriding sandy column. From 30 feet (10m) down to about 180 feet (60m),
are all gravely sand. There is also positive correlation between the sand from the borehole
and soil on top of the sandstone unit.
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Sabinus I. Ibeneme et al.
0.1
DEPTH BELOW THE SURFACE (METERS)
6.6
ORGANIC – RICH TOP SOIL
LATERITIC SAND
IRON-STAINED SAND
10.8
MEDIUM-GRAINED SAND
19.5
GRAVELLY SAND
42
46
MEDIUM-COARSE GRAINED
SAND
GRAVELLY SAND
Figure 15: Geologic Log of the bore hole
6 Conclusion
Subsoil evaluation within a proposed site for civil engineering structure using
geoelectrical and geotechnical methods of investigation were carried out in this research.
The investigation was to provide information on the stratigraphy, nature, structural
disposition and competence of the subsoil. Six (6) Vertical Electrical Sounding (VES)
stations were occupied and this was complemented with geotechnical analysis of six soil
samples collected at all the VES points down to a depth of 2m within the study area.
Quantitative interpretations of the VES curves were carried out using partial curve
matching technique and computer iteration technique. The investigation delineated four
geologic layers which include the Sandy and Organic-rich top soil, Iron-stained sand,
Medium-grained sand and Gravely sand. The topsoil composed mostly of sand and
laterite with resistivity generally greater than 350Ωm. The layer (topsoil) with thickness
varies from 1.4 – 5.8m constitutes the layers within which normal civil engineering
foundation is founded. There are no indications of any major structures such as faults,
fractures and lineaments down to a depth of 40m that could be deleterious to engineering
construction in the area. All the determined geotechnical parameters of the subsoil fall
within the specification recommended for foundation material [22]. From the results, we
conclude that soil sample between 0.0-2.0 meter (0-6.6 feet) are characterized by very
high bearing capacity of values ranging from 381 to 524 KN/M2. The deduction from the
above is that, the topsoil Formation may be rated as relatively good as a foundation
material. The foundation of the proposed civil structure can be hosted by this formation.
7 Suggestion for further Work
In the light of the gulley northwest of the site, we suggest that high rising structures
should not be sited within the northern and northwestern part of the area and filling of the
Geoelectrical and Geotechnical Evaluation of Foundation Beds
93
gulley with materials of equally high bearing capacity such as sandstone or lime-bearing
matter is recommended. Already sliding and slumping have commenced and gulleying
may be advanced if not checked, irrespective of the relatively level topography.
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